William Shawcross is flirting furiously with our photographer, Michele Mossop, as I walk into The Bridge Room at the big end of Sydney, three minutes late and apologetic. “That’s fine," he beams. “I’m just talking to this beautiful woman. She can stay as long as she likes."

The 66-year-old English gent, in town with his third wife, Olga Polizzi, daughter of British hotel legend Lord Forte, is one of the best regarded writers on geopolitics of the past four decades. He has spent the week charming the pants off The Sydney Institute – the current affairs forum brought him to Sydney – on topics as multifarious as
Rupert Murdoch
, al-Qaeda and the Queen Mother.

He has just released books on the latter two, and has well-thumbed copies of them next to him, indexed with scribbled Post-it notes.

Justice and the Enemy has attracted controversy because Shawcross, a youthful Labour Party member turned establishment Tory, seems to advocate the torture technique known as waterboarding for terrorism suspects.

But he is perhaps best known here for penning what fans say is the definitive Rupert Murdoch biography, published in 1992. For research, he made his first trip to Sydney; he has “loved it ever since".

Shawcross has no time for fuss and wants to order lunch straight away. Our waiter describes the main course special of mulloway, which he selects without a second thought. I choose the lamb and cajole him into sharing the octopus entree.

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He shuns the wine list, chuckling: “I drank too much wine last night, and I find anyway that drinking at lunchtime makes me feel terrible by 4 o’clock. But when I was your age at The Sunday Times in the 1970s we drank a bottle each at lunchtime. God knows how we did it."

Shawcross had an “incredibly lucky" childhood growing up in the Sussex countryside with a brother and sister in the ’40s and ’50s.

His mother, Joan, died in a riding accident in 1974. His father, who died in 2003 aged 101, was Lord Hartley Shawcross. He was attorney-general in Clement Attlee’s postwar Labour government, which rebuilt Britain and formed its modern welfare state around Keynesian economics. But he is best remembered as lead British prosecutor of the remaining Nazi leaders at the historic Nuremberg war crimes trials that followed World War II and the Holocaust. His brilliant closing argument, made when William was just two months’ old, undermined any claims that the trials were an exacted vengeance against the defeated Germans.

William listened again to his childhood records of his father’s dignified and haunting prosecution arguments for inspiration for his latest “difficult" book.

“When Obama was having trouble in early 2010 trying to carry out his promise to hold trials of Guantanamo Bay prisoners in New York due to the adverse political reaction, my publisher rang and said: ‘William, your father was the British prosecutor at Nuremberg, so why not write a short, sharp book saying Nuremberg was how we dealt with monsters then, how should we deal with them today?’

“So I love my father and thought he was terrific and thought it would be interesting to do," he pauses before guffawing, “but it was much harder than I expected!"

Justice and the Enemy’s most contentious conclusion is that the terrifying interrogation method waterboarding – to which Shawcross’s contemporary, the late Christopher Hitchens, subjected himself and concluded “with typical elan" was indeed “torture" – is acceptable in some cases.

“I quote Michael Walzer, the American philosopher, about a politician who feels he has to use torture to save lives," says Shawcross, and reads from the book: “His choices are hard and painful and he pays the price not only while making them but forever after.

“And I think you could say that about
George Bush
. He has paid a huge price, and so has the United States, and the question is whether that price was worth paying."

The answer, for Shawcross, seems to be yes. He cites American intelligence officials, and Eliza Manningham-Buller, the former head of MI5, who say intelligence was gained from waterboarding. It was also, he says, limited to three prisoners, not hundreds, as some, including The American Civil Rights Union, have claimed.

But he admits even Manningham-Buller concluded the technique was not justified despite saving lives. So does the US not risk losing the moral high ground and its own humanity by allowing such practices?

“Yes, the risks are awful," he says gravely. “I think we are fighting a really, really serious war, but it’s really important that you understand the risks, that fighting in any improper manner is terribly frightening, and it’s a huge, huge problem."

Isn’t there also a risk of overstretching the comparisons between the Nazis and al-Qaeda? Can the death of 3000 people in 9/11 really be compared with the deaths of 6 million Jews?

“Yes, I think you’re right," he says. “I mean the Nazis were a war machine and al-Qaeda are not a massive war machine on the same scale, that’s true. This is a different kind of threat. It’s more a sort of infection. Al-Qaeda has become sort of an ideology that is brutal and destructive, mostly of Muslims, one has to acknowledge, whether it is in Pakistan or Afghanistan or Somalia or Mali, most of the people killed by Islamist groups are Muslims."

Shawcross has been scorned by some as a Bush apologist but laughs that he thought the left would “hammer" the book far more than they have.

Further relief arrives with our generous half portions of octopus. And I experience Shawcross’s own interrogation method: concise but probing questions, asked with an accidental air of disinterest. Before I know it, he has half my life story.

Shawcross went to Eton, alma mater of Boris Johnson, Prime Minister David Cameron, Chancellor of the Exchequer George Osborne and, when Shawcross was growing up, Prime Minister Harold Macmillan.

“It’s a wonderful school," he says of the institution, the privileged image of which still rubs certain sections of British society the wrong way.

He left Oxford University in 1968 and passed the Foreign Office exam. But a career in the diplomatic service was aborted and his life direction fundamentally changed that summer on holiday in Czechoslovakia, where he experienced the might of the Russian invasion in response to the Prague Spring reforms of Alexander Dubcek.

“I was 22. I decided, after a lot of agonising, that this was so exciting and passionate and awful, what was going on in Czechoslovakia with the Russian suppression, that I wanted to write about it. So I burned my boats with the Foreign Office."

He won a £50 advance from a publisher and later, a promising response to Dubcek: Dubcek and Czechoslovakia 1918-1968.

“I was 24. I was lucky."

He then book-ended a stint at The Sunday Times with a stream of major works on international and global politics. The most notable, in 1979, was Sideshow: Kissinger, Nixon and the Destruction of Cambodia. It was nominated for a Pulitzer Prize.

In the late ’80s he turned his attention to business, Australia and Rupert Murdoch, who had just become a US citizen. Murdoch: The Making of a Media Empire got him exclusive access to the man himself and his family. It was damned as hagiography by the mogul’s enemies but simultaneously offended those close to the News Corp chairman.

“Murdoch was more sanguine about it," he smiles mischievously. “At a press conference here in Australia he said: ‘William Shawcross must be a reasonable journalist: he’s upset my mother, my wife and all my enemies’. It was a very nice quote, which of course I put on the back of the paperback."

Shawcross insists the 81-year-old is “more sinned against than sinning" despite the phone-hacking scandals that have rocked his British newspaper operations. British journalism, says Shawcross, has the proprietor to thank for its survival to this day because he smashed the print unions in the 1980s.

But what of British novelist and critic (and former Fleet Street political editor) Robert Harris, who accused Shawcross of being “seduced" by Murdoch? “Oh well," he shrugs, “probably true. Rupert is a great seducer. He is so charming, he is really charming and, it’s amazing, so was his mother. I’m making light of it because I don’t care what Robert says. He’s entitled to it. But I do admire Rupert, I think he’s done extraordinary things.

“But," he remembers to add, “clearly something terrible went wrong at the News of the World in the last few years and the price is awful, probably several
News Corp
employees will go to prison and it’s been incredibly damaging to Murdoch himself."

More broadly, he says he is “terrified" that newspapers could go out of business because fragmented internet media will not hold governments as accountable.

As our mains arrive I ask about his memories of Dame
Elisabeth Murdoch
, who died this month, aged 103, and whose funeral was held at St Paul’s Cathedral in Melbourne with her family in attendance.

He was upset by a letter from her describing her offence at the book but reveals she later came to visit his late mother’s garden in Sussex.

“It was very sweet of her. She was an astonishing and adorable woman. A great matriarch of Melbourne and Victoria as well as her family. That generation produced some extraordinary people, it was pretty much the same generation as the Queen Mother."

Which brings us to the current crop of royals – and that prank phone call by DJs at Sydney’s 2Day FM to the hospital caring for a sick and pregnant Duchess of Cambridge. Nurse Jacintha Saldanha committed suicide days after transferring the call to a colleague, who described the Duchess’s condition in detail.

“It’s extraordinary the two young DJs did it, but also that the station considered, ‘Shall we put this out?’ and thought they should," he says.

As for the House of Windsor, he is an even more staunch defender of it than he is of Murdoch. “I think it’s a good system. Whether Australia is going to remain [a part of the Commonwealth] forever, I don’t know, but it’s good for us and has worked very well and we’ve had some extremely good heads of state who’ve been kings and queens."

He quotes conservative English philosopher Roger Scruton, who dubbed the monarchy the “light above politics". “It’s a nice phrase and I think that’s what the Queen has been, she’s had 10 prime ministers and neither Cameron nor Blair were born when she came to the throne. She represents the nation to itself and does it very well. Of course you can have a republic and most countries are and work perfectly properly, although not all."

But how can he justify the royalist philosophy that members of the monarchy are entitled by dint of their birth? “I know," he parries, smiling. “It’s not logical. It’s a romantic institution . . . and long may it survive, as far as I’m concerned!"

My lamb is succulent. Shawcross’s mulloway is “delicious". As his short black arrives, he’s already thinking of taking his Australian friend Sarah Burns, wife of Cornwall-based celebrity chef Rick Stein, to The Bridge Room when she lands in Sydney. Shawcross visits Cornwall (where his wife, Olga Polizzi, has one of her hotels) as often as he can, in between working in London, where they live in Paddington.

His son, Conrad, by his first wife, writer and art critic Marina Warner, is making his name as an artist. Shawcross himself studied at St Martins Art School, and on his last visit to Australia three years ago visited an exhibition of Conrad’s work at the “fantastic" Museum of Old and New Art in Hobart.

His daughter Eleanor, daughter of his second wife, Michal Levin, is tied to Conservative Party royalty. She is married to Tory life peer Baron Wolfson, chief executive of clothing retailer Next, and is herself a senior adviser to George Osborne.

Shawcross, who has campaigned on conservation in Cornwall, was appointed Commander of the Royal Victorian Order last year. His new part-time role as chairman of Britain’s Charity Commission bothers some on the left, who say his political views are too strong.

“Some friends said to me it’s a poisoned chalice and I should never dream of sipping from it, and others said: ‘It’s about time you did some public service, William’."

If he succeeds at his first “real job" he’ll look like a shoo-in for a knighthood.